The present invention relates to a numerical controller of a grinding machine for machining a non-circular workpiece such as a cam and the like.
In a conventional grinding machine of this kind, the position of a grinding wheel is controlled in synchronism with the rotation of a spindle in accordance with profile data which defines the relation between the rotational angle of the spindle and the position of the grinding wheel. The profile data is obtained as a changing component of the grinding wheel position which varies with respect to the rotational angle of the spindle. The changing component indicates a relative moving locus of the center axis of the grinding wheel when the grinding wheel makes a round tangentially to the periphery of the non-circular workpiece such as a cam and the like. When the wheel diameter decreases on account of wear, the changing component is varied, since the contact point between the wheel and the cam does not stay on the plane formed by the spindle and the rotary shaft of the wheel and changes depending on the wheel diameter as well as the rotational angle position of the workpiece. Accordingly, in order to accurately grind workpieces, the profile data must be, if the present wheel diameter is varied, recalculated from lift data and the present diameter of the grinding wheel. However, if the profile data is calculated whenever the wheel diameter decreases by a small amount, it is time consuming for machining and not realistic.
The present wheel diameter, in practice, reduces gradually by periodical wheel dressings. When the wheel diameter reduced by the wheel dressings reaches a marginal diameter whereat the finished shape error of the workpiece exceeds a tolerance if the profile data is not recalculated, the operator must give commands to the numerical controller to recalculate the profile data from the present diameter of the grinding wheel and lift data.
However, since the marginal diameter of the grinding wheel, at which the finished shape error of the workpiece exceeds a tolerance, changes depending upon the profile of the non-circular workpiece such as the cam and the present wheel diameter, the marginal diameter or the timing of recalculating of the profile data can hardly be determined uniformly.
Therefore, in the past, the timing of recalculating of the profile data has been decided by experience and intuition of the operator.
Accordingly, such problems are encountered that the machining cycle time increases on account of overdoing renewal of the profile data more than necessary, or conversely, the finished shape error of the workpiece may exceed the tolerance on account of delay of recalculation timing of the profile data.